Some chlorinated compounds, and in particular polychlorinated biphenyls (PCBs), TCE and related VOCs, have become a difficult environmental problem in that extremely minute levels are believed to-present a health risk. PCBs are a class of 209 discrete chemical compounds known as "congeners", in which one to ten chlorine atoms are attached to biphenyl. PCBs were widely used for several decades due to their superior properties of chemical and physical stability, heat resistance and high electrical resistance. They were used in heat transfer systems, hydraulics/lubricants, transformers, capacitors, plasticizer applications and petroleum additives, just to name a few.
The chemical and physical stability, viewed once as an asset of the PCB, is now recognized as an environmental liability since the PCBs do not readily degrade after disposal. Now the use of PCBs is regulated in the United States under the Toxic Substances Control Act (TSCA), PL 94-469 (U.S. Congress, 1976). This law is administered by the United States Environmental Protection Agency (EPA). Various rules and regulations have been promulgated concerning the production, use and disposal of PCBs. Other countries have passed similar legislation, making the control of PCB use and disposal a world-wide concern. While PCBs are presently not in wide scale production, it has been estimated that up to 1.3 billion pounds were produced world wide through the year 1976.
Given the chemical and physical stability of PCBs, it has been of increasing concern to regulators to monitor and screen samples of various types for PCB contamination. In general, while the specifics of the various national laws may differ, there is a common interest in determining the presence of PCBs in the environment. Determination techniques used in the past include gas chromatography, thin-layer chromatography, and high-performance liquid chromatography. Non-chromatographic techniques include nuclear magnetic resonance (NMR) spectrometry, infrared (IR) spectrometry, and immuno-assays.
"Screening" techniques are determinations characterized by speed and/or simplicity of methodology and apparatus. Typically, samples are screened where immediate analysis is needed, such as analysis in the field or during an incinerator trial burn to make sure that the PCBs are being destroyed. In general, the analysis of PCBs generally requires selectivity and sensitivity. Even after cleanup of a PCB-contaminated site, PCBs are usually at ultra-trace levels in field samples, mixed with other halocarbons, lipids, etc. The levels of PCBs typically found in water, soil, tissue, food, biota and other matrices of interest are in the parts per billion (ppb) range. Most current measurement techniques for PCBs require the aforementioned chromatographic separation techniques, which are not practical for routine analysis in the field. A review of the state of the art in PCB detection can be found in Analytical Chemistry of PCBs by M. D. Erickson (Butterworth Publishers, 1986). As described therein, packed column gas chromatography (GC), thin-layer chromatography (TLC), and high-performance liquid chromatography (HPLC) have been used to provide data on total PCB contents in samples. Packed column GC/ECD is the common method for quantification of PCBs such as AROCLORS made by the Monsanto Corporation in the American National Standards Institute (ANSI) procedures. In this procedure, the PCBs are quantified against an AROCLOR standard using the largest peak, or a secondary peak if necessary. Typically, this procedure was used to determine PCBs in sediments and soils.
If congener-specific determination is required, high-resolution gas chromatography (HRGC), which uses fused silica capillary columns, would be the preferred technique. High-resolution gas chromatography has been used for the analysis of PCBs in transformer fluids or waste oils.
Various mass spectrometry (MS) techniques, including electron impact MS, chemical ionization MS, coupled MS/MS, etc., have been used to analyze complex PCB samples. Methods involving perchlorination of the biphenyl ring of the PCB congeners have been used in the determination of PCBs. One limitation of the perchlorination approach is due to the fact that biphenyl can also be perchlorinated, thus leading to erroneously high blank levels.
Thin-layer chromatography (TLC) has been used in the analysis of PCBs. Detection using this technique has involved spraying the plates with silver nitrate followed by UV irradiation and fluorescence. See R. H. DeVos and E. W. Peet, Bul. Envir. Contam. Toxicol., 6 (2), 164, 1971, for UV irradiation and J. Stahr, Liq. Chromatogr., 7, 1393, 1984, for fluorescence. Two dimensional TLC has been used for PCB analysis, as described by N. V. Fehringer and J. E. Westfall in J. Chromatogr., 57, 397, 1971. Photo-degradation of PCBs is a previously known process and fluorescence following UV excitation has been reported. See The Chemistry of PCBs by O. Hutzinger et al., R. E. Krieger Publishing Co., 1983.
Sensitized room temperature phosphorescence (RTP) has been used as the detection method for HPLC in PCB analysis. The method is based on the transfer of triplet energy of the analyte molecule (donor) to biacetyl (acceptor) and detection of sensitized RTP of biacetyl in liquid solutions. See T. Vo-Dinh, Room Temperature Phosphorimetry For Chemical Analysis, Wiley Publishers, 1984. The RTP method has recently been applied to PCB analysis.
Volatile organochloride (VOC) contaminants and co-contaminants at many industrial plants can be found at many industrial plants and federal sites. Many sites of the U.S. Department of Energy (DOE) Are contaminated with mixtures of VOCs including trichloroethylene (TCE), perchloroethylene (PCE), and carbon tetrachloride. TCE has been widely used in the past as a common industrial solvent (degreasing agent). There has been a concern that TCE has contributed to the ozone depletion in the atmosphere. TCE is considered a significant environmental pollutant because laboratory bioassays have shown that its metabolized products can cause nephrotoxicity and nephrocarcinogenicity in laboratory animals. There is thus a strong need to develop a sensor that can detect liquid or vapor of TCE and related chlorinated compounds in-situ.
The analysis of TCE and chlorinated compounds (such as chlorinated solvents and volatile organochlorides) generally requires selectivity and sensitivity. TCE are usually at ultra-trace levels in field samples, mixed in with other halocarbons, hydrocarbons, etc. The levels of TCE typically found at industrial plants, DOE sites, or waste sites are in the part-per-billion (ppb) range to part-per-million (ppm) range.
There is a continuing need for a chemical sensor capable of detecting TCE and VOCs in liquid and vapor samples under field conditions.